Comparison of multiple methods for the determination of sulphite in Allium and Brassica vegetables

Sulphites are a family of additives regulated for use worldwide in food products. They must be declared on the label if they are present in concentrations greater than 10 mg kg^-1, determined as sulphur dioxide (SO₂). The current US regulatory method for sulphites, the optimised Monier-Williams method (OMW), produces false-positive results with vegetables from the Allium (garlic) and Brassica (cabbage) genera due to extraction conditions that are thought to cause endogenous sulphur compounds to release SO₂. Recently, modifications to the OMW method (2x MW) were published that reportedly reduced this false-positive in garlic. However, no other vegetables from these genera have been investigated. In addition, an LC-MS/MS method was developed for sulphite analysis, but it has not yet been tested with these problematic matrices. Ten vegetable species were analysed using these sulphite methods (OMW titration, OMW gravimetric, 2x MW and LC-MS/MS) to determine the false-positive rate. Sulphite concentrations > 10 mg kg^-1 SO₂ were observed with the OMW analyses. The 2x MW method reduced the measured concentration in unsulphited samples to ≤ 10 mg kg^-1 SO₂ for all matrices analysed. The LC-MS/MS method showed concentrations < 10 mg kg^-1 for the Brassica samples, but only displayed a slight reduction in the Allium matrices. Spiked recovery studies were conducted to determine if these methods can detect added sulphite. The 2x MW had recoveries of 17% and 42% for water and fresh garlic, respectively, and the LC-MS/MS had recoveries of 108%, 125%, 116% and 107% for water, fresh garlic, roasted garlic, and hummus, respectively. The low recoveries of the 2x MW may indicate that sulphur compounds cannot be properly quantified with this method. The ability to eliminate false-positives will enable accurate determination of added sulphite to ensure compliance with sulphite labelling requirements.